This picture should give you a good idea
what the round stair tread and riser looks like once it's completed.
Simply nail the plywood riser strips onto the plywood nailers that are
attached to the concrete floor and to the bottom side of the stair tread
and you're done.

Make sure that you measure around the stair tread nosing to make sure
that it's the same all the way around. If the plywood nailers weren't
made correctly, you could have a problem with your stair tread nosing.